We propose to examine in detail the discrete waves or "quantum bumps" that appear to compose the afterpotentials present in many invertebrate photoreceptors. The photoreceptors of Drosophila are well suited for such a study because their prolonged depolarizing afterpotential (PDA) is particularly long-lived, and because the bump processes underlying their light-coincident receptor potential (LCRP) have been well studied. The questions we seek to answer are whether the PDA, like the LCRP, is a summation of bumps and, if so, whether the bumps that constitute the PDA are indistinguishable from those that make up the LCRP. We intend further to describe the post-stimulus timecourse of the PDA in terms of the underlying bump processes. The bump processes of the wild type fly will be compared with those of several PDA-defective mutants isolated in our laboratory. In this way we hope to define the sequence of events leading to bump generation during PDA or LCRP, and which step or steps in this sequence have been affected in each mutant. It is our intention to synthesize these finding in the form of a quantitative model of the phototransduction process.